In general, a wireless communication system defines a guard band to avoid mutual interference between channels (or Frequency Assignments (FAs)). For example, in an Orthogonal Frequency Division Multiplexing (OFDM)-based system, the guard band is used so that a spectrum of an OFDM signal has a ‘brick wall’ pattern. In addition, by transmitting a ‘0’ (i.e., no signal) on a plurality of left and right subcarriers, a signal component acting as interference to an adjacent frequency band is decreased after performing Fourier transform.
FIG. 1 illustrates a frequency domain in a conventional OFDM-based communication system.
Referring to FIG. 1, the OFDM-based communication system performs communication by using an FA that is a channel frequency assignment unit (i.e., bandwidths of 5 MHz, 10 MHz, and 20 MHz), and is assigned with multiple FAs 101 and 111. Each of the FAs 101 and 111 consists of a plurality of subcarriers 102 and 112. An fc1 101 is a center frequency of the FA 101. An fc2 110 is a center frequency of the FA 111. A guard band 130 is provided to avoid interference between the FA 101 and the FA 111.
Meanwhile, a standard or the like is being revised so that a communication system is evolved to provide a high-speed data service in comparison with a legacy system or to address an implementation issue. In such a system evolution process, various systems can coexist in the same area according to compatibility the legacy system. For example, a new system that uses an FA of 20 MHz and is further evolved from the legacy system using an FA of 10 MHz, may be deployed in an area where an OFDM-based wideband wireless communication system is installed. The legacy system supports only a Mobile Station (MS) that has a single bandwidth in single FA. That is, the legacy system using the FA of 10 MHz has a structure that can support only an MS using an FA of 10 MHz. Therefore, in order to support a new MS having a greater bandwidth, such as an MS using an FA of 20 MHz, a new FA having a bandwidth of 20 MHz is necessary.
Accordingly, researches on a frequency overlay technique, which uses conventional multiple FAs without assigning a new frequency band, are being introduced. In the frequency overlay technique, the conventionally used multiple FAs are simultaneously used to support a required new FA. For example, in a state where two FAs of 10 MHz are previously used, a service using an FA of 20 MHz can be provided without additional frequency assignment when the two FAs of 10 MHz are subjected to a frequency overlay operation. A guard band defined to avoid interference between two FAs of 10 MHz may be used as a data region in the frequency overlay operation in order to increase bandwidth efficiency.
In this case, as shown in FIG. 1, a spacing between the center frequencies, i.e., the fc1 100 and the fc2 110, of the two adjacent FAs 101 and 111 may not be an integer multiple of a subcarrier spacing. For example, in the Institute of Electrical and Electronics Engineers (IEEE) 802.16e standard, if an FA of 10 MHz is used, a sampling frequency is 11.2 MHz and a subcarrier spacing is 10.9375 kHz. When the spacing between the two adjacent FAs is 10 MHz, it is not an integer multiple of the subcarrier spacing, and thus, disparity occurs in a raster between the adjacent FAs. In such a case, the guard band 130 is required since mutual interference occurs when using a subcarrier included in the guard band 130 located between the two FAs. Accordingly, since the guard band cannot be used in the frequency overlay operation, there is a disadvantage in that spectral efficiency decreases.